1. Field of the Invention
The present invention relates to a semiconductor switching element, a variable inductor, and an electronic circuit device having a circuit configuration incorporating the switching element and the variable inductor therein. Specifically, the invention relates to a switching element having a continuity control region in which length of a continuity region for switching can be varied, and a variable inductor using the switching element to be able to vary the inductance.
2. Description of the Related Technology
In a variable matching circuit, variable filter circuit, or variable oscillation circuit, an inductance value or a capacitance value of an LC resonance circuit is appropriately changed, thereby variable operation of each function is achieved. Generally, when the capacitance value is changed, a variable capacity diode is used. Since the variable capacity diode uses reverse bias capacitance characteristics of a semiconductor junction, the capacitance value can be continuously changed in the diode. In addition, since the variable capacity diode can be formed using a typical semiconductor process, reduction in cost can be achieved. On the other hand, an effective element for changing the inductance value is hardly found. Therefore, the variable matching circuit, variable filter circuit, or variable oscillation circuit often operates while changing only the capacitance value. However, if each circuit can operate while further changing the inductance value, a frequency variation range of a circuit is widened, and thus a circuit function that has been achieved by a number of resonance circuits in the past can be achieved by a smaller number of resonance circuits, and consequently reduction in size of a circuit can be achieved. From such a reason, a variable inductor, of which the inductance value can be continuously changed, is desired.
As variable inductors in the related art, variable inductors disclosed in JP-A-8-162331 and JP-A-2004-140165 are given. JP-A-8-162331 discloses a variable inductor including a plurality of loop-like wiring layers having open ends at a lower part of a spiral inductor, a field-effect transistor as a switch for opening/short-circuiting each of the open ends, and an insulating film formed between the spiral inductor and the wiring layers; wherein the switch is opened and closed to vary an inductance value. JP-A-2004-140165 discloses a variable inductor including a first inductor having a predetermined inductance value, a second inductor having a predetermined inductance value, the second inductor being formed having mutual inductance with respect to the first inductor, and a variable resistance connected between terminals of the second inductor; wherein a resistance value of the variable resistance is continuously changed, thereby an inductance value between terminals of the first inductor is continuously varied.
However, the following inconveniences or problems exist in the background art as above.                (1) In the variable inductor of JP-A-8-162331, since an inductance value is digitally changed, it cannot be continuously changed. Therefore, accurate changing of the inductance value cannot be achieved.        (2) In the variable inductor of JP-A-2004-140165, while the inductance value can be continuously changed, since an induced current flowing in a coupling coil is consumed by a resistive element, a Q value is significantly reduced.        
Features of the background art disclosed in JP-A-8-162331 and JP-A-2004-140165 are summarized as shown in the following Table 1.
TABLE 1Is inductanceIs QcontinuouslyvalueMethodchanged?high?JP-A-8-162331ON/OFF of coupling circuitNoYesJP-A-2004-140165Increasing or decreasingYesNoresistance value of couplingcoil